One minute, Jen-Ho Tseng was named the Cubs' minor-league pitcher of the year Wednesday.

Seemingly the next moment, the 22-year-old right-hander from Taiwan was named the team's starting pitcher for Thursday's game against the Mets.

Tseng will make his major-league debut in place of Mike Montgomery, who returned to the bullpen and will be there "going forward," according to manager Joe Maddon.

"I said, 'I guess you're in town to accept an award,'" Maddon said. "I said, 'How about you start (Thursday) night's game instead?' He didn't even blanch."

Tseng was in uniform before Wednesday night's game at Wrigley Field, two days before he's to accept his second minor-league pitcher of the year award from the organization, along with the team's minor-league player of the year, Victor Caratini.

But Maddon cautioned that Tseng might be here one minute and gone the next, depending on how he performs and how Jake Arrieta's right hamstring is feeling.

Montgomery had been filling in for Arrieta in the rotation, but the struggles of left-handed reliever Justin Wilson, at least in part, highlighted Montgomery's need as a long reliever.

"It could be one and done," Maddon said of Tseng's start. "We'll see how it plays out. Sometimes you catch lightning in a bottle."

Maddon said there's "no absolute ending to this thing yet," in reference to when Arrieta may return to the rotation. He said the team will know more after Arrieta throws again, which had not been scheduled as of Wednesday evening.

The Cubs signed Tseng as an international free agent in 2013. He was 13-4 with a 2.54 ERA in 1451/3 innings between Double-A Tennessee and Triple-A Iowa this year.

As an 18-year-old in 2013, Tseng was the only high school player on Taiwan's team in the World Baseball Classic.

The 6-foot-1, 210-pounder has five pitches and his velocity hovers in the high-80s to low-90s. His style drew comparisons from Maddon to Cubs starter Kyle Hendricks.

Tseng found himself not far from Chicago in 2014, when he played for the Class A Kane County Cougars, going 6-1 with a 2.40 ERA in his first pro season.

Tseng told the Des Moines register in July that meticulous preparation has allowed success on the minor-league level.

"I still tell myself, 'You have to keep it going; you can't stop right here,'" he told the paper.

The Cubs designated right-handed pitcher Pierce Johnson for assignment to make room for Tseng on the roster. Tseng will wear No. 39.

Simulated session: Right-handed reliever Koji Uehara threw a simulated session Wednesday to test out an infection in his right knee. Maddon said Uehara will be re-evaluated Thursday. He has not pitched since Sept. 2.